The invention relates generally to resource allocation in a wireless code division multiple access communication system. More specifically, the invention relates to controlling user equipment access attempts for communicating over a random access channel in a wireless code division multiple access communication system.
FIG. 1 depicts a wireless spread spectrum Code Division Multiple Access (CDMA) communication system 18. A base station 20 communicates with user equipments (UEs) 221-22N in its operating area. In a spread spectrum CDMA system 18, data signals are communicated between UEs 221-22N and the base station 20 over the same spread spectrum. Each data signal in the shared spectrum is spread with a unique chip code sequence. Upon reception, using a replica of the chip code sequence, a particular data signal is recovered.
Since signals are distinguished by their chip code sequences (code), separate dedicated communication channels are created using different codes. Signals from the base station 20 to the UEs 221-22N are sent on downlink channels and signals from the UEs 221-22N to the base station 20 are sent on uplink channels.
In many CDMA systems, a random access channel (RACH) is used and is capable of carrying packets of data from multiple UEs 221-22N. Each packet is distinguishable by a combination of time slot and code.
The transmission is time divided into repeating frames having time slots, such as fifteen time slots per frame. When a packet is transmitted over the RACH, it may last for multiple frames.
A typical UE RACH access attempt is as follows. Prior to communicating over the RACH, a UE 221 transmits an access signal to the base station 20 to access the RACH. One type of access signal uses a preamble code (preamble). The UE 221 repeats the preamble while incrementally increasing transmission power levels. The UE 221 repeats transmission of the preamble unit a response from the base station 20 is received or until a maximum number of repetitions is reached.
In response to receiving the preamble, the base station 20 determines whether the UE 221 may utilize the RACH. This utilization determination may be based on the availability of the RACH channel, uplink interference levels or RACH loading. If the access attempt is successful, the base station 20 transmits an acknowledgment signal (ACK) to the UE 221. In response to the UE 221 receiving the ACK, the UE sends an uplink packet over the RACH. If the RACH is not available, the base station 20 transmits a negative acknowledgment signal (NAK) to the UE 221. Receiving a NAK or reaching the maximum number of repetitions are unsuccessful access attempts requiring the UE 221 to reattempt access at a later time.
The period of time between access attempts is critical to a system's performance. If the period between access attempts is too long, the RACH will be underutilized. If the period is too short, many UEs 221-22N may repeatedly request access resulting in service interruptions.
One approach for controlling UE re-access attempts is to use a fixed backoff parameter. The UE 221 will reattempt access for a period of time based on the backoff parameter. The backoff parameter represents a deterministic wait period for an access reattempt. A problem with a fixed backoff parameter is that it can not be adjusted in response to the cell loading. Accordingly, during periods of light loading, the RACH may be underutilized and in periods of high loading service interruptions may result.
Another approach is a rule based approach. The UE 221 analyzes its prior access attempt statistics. Based on the access statistics, the UE 221, applying predetermined rules, determines a backoff parameter. To illustrate, if the UE 221 had many unsuccessful access attempts, the period between accesses is increased. Since the UE's prior access attempts may not represent current conditions, this approach is not optimal.
Another approach is to broadcast a backoff parameter over a broadcast channel (BCH). The backoff parameter is based on the RACH's loading, uplink interference level and other factors. The backoff parameter, Ubbch, is used to derive a backoff wait period at a time n, B(n), such as by equation 1.B(n)=2Ubbch   Equation 1
Due to delays in processing and transmitting the backoff parameter, the backoff parameter may not represent current conditions which is not optimal.
Accordingly, it is desirable to have other approaches for controlling UE access reattempts.